1. Field of the Invention
The present invention relates to a process for manufacturing the elastomeric sleeve of a joint for electrical cables.
In particular, the present invention relates to a process for manufacturing the extruded insulating element of the elastomeric sleeve of a joint for extruded electrical (power) cables.
Furthermore, the present invention relates to an apparatus for manufacturing the elastomeric sleeve of a joint for electrical cables, said sleeve comprising an extruded insulating element.
In particular, the present invention relates to an apparatus for manufacturing the extruded insulating element of said elastomeric sleeve.
2. Description of the Related Art
Generally cables for conveying or supplying energy, in particular for conveying or supplying medium-voltage or high-voltage energy, comprise, from the inside towards the outside of the cable: a metal conductor, an inner semiconductive layer, an electrical insulating layer, an outer semiconductive layer, a metal screen (usually made of aluminium, lead or copper) and an external polymeric sheath. The predetermined sequence: metal conductor, inner semiconductive layer, insulating layer and outer semiconductive layer is generally known with the term of “cable core”.
In order to join two electrical cables, for example of the single-core type, over a portion of defined length, the ends of both the electrical cables are stripped so as to expose the constitutive elements thereof in a staggered way.
In the case the joining operation is performed between two electrical cables of the multi-core type, for example double-core or triple-core type, the procedure described hitherto is repeated for each single phase of each cable.
Subsequently to said step of stripping, the joining operation comprises the steps of forming an electrical connection between the cables conductors disposed end-to-end and of providing an elastomeric sleeve to be fitted upon and tightened over the joined ends of said cables.
Preferably, the electrical connection is formed by welding or by using a compression clamp or the like.
Generally, said elastomeric sleeve has a form which is substantially cylindrical in its central portion and of frusto-conical shape at its ends so as to provide an optimum mechanical connection between the joined cables and the sleeve itself.
The elastomeric sleeve comprises a plurality of radially superimposed elements intended to restore the electrical and mechanical connection between exposed layers of a first cable section and corresponding exposed layers of a second cable section.
Starting from its innermost portion, said elastomeric sleeve generally comprises:                an electric field-control element which is generally known with the term of “electrode”;        an electrical insulating element which surrounds said electrode, and        at least one semiconductive element which is positioned radially external to said electrical insulating element.        
The electrode is a voltage distribution element which is positioned, generally as a tubular shield, in correspondence of the joined ends of the cables and partially covers the insulating layers thereof. The electrode is generally made of a semiconductive material and creates a sort of Faraday cage at constant potential to annul the effects due to shape irregularities of the electrode.
The semiconductive element of the elastomeric sleeve has the function of connecting the outer semiconductive layers of the cables so that the continuity of the outer semiconductive layers of said first and second sections of said cables can be restored.
Generally, the semiconductive element comprises two cup-shaped stress control screens, which are positioned at the axial ends of said electrical insulating element, and an insulation screen, which surrounds the electrical insulating element and is positioned between said stress control screens.
Generally, the elastomeric sleeve is manufactured separately and supplied fitted, in an elastically-dilated condition, on a hollow tubular support made of rigid plastic which is successively removed so that the sleeve elastically contracts and grips the cable sections in the joining zone.
This support may be obtained, for example, from a strip-like element helically wound to form a plurality of adjacent spirals fastened together so that, when a pulling force is exerted on a free end portion of said strip-like element, the tubular support is able to collapse, due to gradual separation of the spirals, and to allow the correct positioning of the sleeve. This sleeve is of the cold-retractable type. Embodiments of said supports are described, for example, in the documents EP-541,000; EP-735,639; EP-547,656; EP-547,667 in the name of the Applicant.
Alternatively, the sleeve may be made of heat-shrinkable materials, thus producing the so-called heat-shrinkable sleeves described, for example, in the document U.S. Pat. No. 4,383,131.
Alternatively, as disclosed in document EP-149,032 in the name of the Applicant, the sleeve can be positioned by means of a rigid tubular support, whose cavity has diametrical dimensions grater than the outer dimensions of the two cables to be joined, which cooperates with a device comprising rigid plates and bars the actioning of which causes a sliding movement between the outer surface of the tubular support and the cavity of the sleeve so that a uniform radial contraction of the sleeve takes place in correspondence of the junction zone of the two cables.
Therefore, the joining operation comprises the step of inserting the sleeve, fitted on the tubular support, on the end to be joined of one of the cables before the abovementioned step of electrically connecting the conductors of said cables.
Successively, the sleeve is positioned in correspondence of the joining zone and the tubular support is removed.
Moreover, since generally a joint also comprises an element intended to restore the metal screen of the cables to be spliced, the joining operation further comprises the step of applying a metallic strip, such as, for example, a tin-plated copper strip, starting from the exposed metal screen portion of the first section of a cable and terminating on the exposed metal screen of the second section of the other cable.
Finally, since a joint generally also comprises an external polymeric sheath suitable for restoring the external mechanical protection of the cable, the joining operation further comprises the step of fitting said sheath in the joining zone, in a position radially external to the aforementioned sleeve, so as to protect the underlying elements of the joint from coming into contact with moisture and/or water from the outside.
Said sheath may be of the heat-shrinkable type or of the cold-shrinkable elastic type or may be obtained by means of a strip-forming step, which may also be combined with the use of suitable mastic sealants.
In case said sheath is of the heat-shrinkable type or of the cold-shrinkable elastic type, said fitting step comprises the step of inserting said sheath on one end of one of said cables to be spliced, said step preceding both the positioning of the tubular support carrying the elastomeric sleeve and the formation of the electrical connection between the cables conductors.
In accordance with further operating methods, restoration of the external mechanical protection of the spliced cables may also be achieved by using several sheaths, for example three in number, arranged so that one pair of sheaths is fitted onto the aforementioned frustoconical portions of said joint and a further sheath is fitted onto the substantially cylindrical portion of the latter.
Methods for manufacturing a joint are described, for example, in documents EP-379,056; EP-393,495; EP-415,082; EP-199,742 and EP-422,567 in the name of the Applicant.
Document JP 10224937 discloses a method for obtaining the electrical insulating element of a joint in correspondence of the joining zone of two electrical cables. Said method comprises the step of injecting a plastic material into a metallic mould, after positioning thereinto the electrically connected cables provided with a high voltage shielding electrode at said joining zone, and the step of moulding said insulating element. Said method further comprises the step of cutting the ends of the cylindrical block member obtained from said moulding step so that a specific shape (i.e. a frusto-conical shape) is given to the insulating element.
Further technical solutions for extrusion moulding the insulating element of a joint are described, for instance, in documents JP 8280115; JP 3280374; JP 5292624; JP 3167773; JP 5859030; JP 5859029; JP 5859027; U.S. Pat. No. 4,377,547; U.S. Pat. No. 4,241,004; U.S. Pat. No. 3,846,578. According to said documents the extrusion process comprises the step of injecting the insulating material into a moulding cavity having the shape of the desired insulating element. For example, documents JP 8280115 and JP 3280374 disclose the use of pressure regulating valves so that a uniform pressure distribution can be reached within the moulding cavity and the generation of voids inside of the insulating material is prevented. Document JP 5292624 discloses metal dies provided with a plurality of injection holes according to a predetermined pitch along the longitudinal direction of the mould.
Document U.S. Pat. No. 3,880,557 discloses a moulding apparatus suitable for moulding the insulating element of a joint for electrically connecting two cables, preferably two high voltage cables. Said apparatus comprises an upper plate and a lower plate, provided with upper and lower moulds respectively which define a moulding cavity inside of which the insulating material is introduced by passing through an injection unit possessed by the upper plate.
The manufacturing of the insulating element of a joint is particularly critical since any defect in the insulating material, such as disuniformities or entrapped air, can originate microcavities inside the insulating element of the joint. Said defects cause the electric strength of said insulating material to decrease so that the probability of formation of electrical discharges within the joint insulating element remarkably increases. As a consequence, the risk of breakdown of the latter during the service of the joint remarkably increases too.
Furthermore, since said defects decrease the electric strength of the insulating material, the insulating element of the joint can breakdown at an electric field gradient lower than the expected one which can be withstood by the insulating material per se.
Said aspect is critical especially in case high voltage joints are considered. In fact, since the latter usually involve high electric field gradients and great thicknesses of the insulating elements, the risk of breakdown is particularly high, said risk increasing with the thickness of the insulating element and, thus, with the maximum voltage the joint is designed for.
It is known in the art that most part of the electrical discharges which originate in a joint generally develops at the axial ends of the electrode in proximity of the tips thereof where the concentration of the flux lines of the electric field is particularly high and the maximum electric field gradient takes place.
FIG. 23 partially shows a typical distribution of the flux lines 100 of the electric field in proximity of a tip 200 of an electrode 300.
According to said distribution it could be expected that the path of an electrical discharge, developing within the insulating material of a joint, follows the electric field gradient, thus being perpendicular to the flux lines of the electric field, since this represents the shortest path which can be covered by the electrical discharge.
However, the Applicant has noted that the path of the electrical discharge generally does not follow said calculated path but a more complex one. In fact, the Applicant has noted that the path of the electrical discharge is remarkably influenced by any defects present in the insulating material.
This aspect is particularly critical since it means that the electrical discharge moves towards the most defective zones of the insulating material and thus the breakdown of the insulating element of the elastomeric sleeve can occur at voltages even remarkably lower than the expected ones.
The Applicant has perceived that this phenomenon is correlated with the extrusion method that is used for manufacturing the insulating element.
In particular, the Applicant has perceived that this phenomenon is correlated with the method according to which the insulating material is introduced into the mould that is used for manufacturing said insulating element.
As described hereinabove with reference to the known prior art methods for manufacturing the insulating element of an elastomeric sleeve, the insulating material is generally introduced (by extrusion or by injection) into a mould through at least one inlet opening, where the electrode and the stress control screens have previously been positioned on a mandrel.
According to said methods, each portion of insulating material entering the mould is advanced thereinto by the pushing action of the successive portions until the filling of the mould is completed. In other words, the insulating material which is the first to be introduced into the mould is pushed along the mould by the insulating material which successively enters thereinto through the one or more inlet openings of the mould.
The portion of insulating material entering the mould at a given time t0 gives rise to a flow line which, due to the successive portions of insulating material that enter the mould at successive times t>t0, becomes substantially parabolic in shape since the velocity of the portion advancing along the mould is lower in correspondence of the walls of the mould, of the electrode and of the mandrel than the velocity of the portion in correspondence of the central zone comprised among the walls of the mould, of the electrode and of the mandrel.
The Applicant has perceived that the path of an electrical discharge preferably follows one or more of said flow lines or the welding zone of two or more of said flow lines. As it is known in the art, weld lines form whenever advancing melt fronts meet (e.g. see “Principles of polymer processing”, Zehev Tadmor, Costas G. Gogos—Wiley-Interscience Publication, 1979, page 594).
Furthermore, the Applicant has perceived that the above mentioned methods of filling a mould with an insulating material for obtaining the electrical insulating element of an elastomeric sleeve give rise to significant anisotropies within the insulating material. Said anisotropies are at least partially due to the fact that each portion of insulating material is caused to advance along the mould by the portions of insulating material entering the mould at successive time instants. It is known that rubber products can exhibit anisotropy and heterogeneity of their physical properties due to molecular orientation which is caused by flow conditions (e.g. see “Mold-flow-induced anisotropy in nitrile rubber”, W. V. Chang, P. H. Yang, R. Salovey—Rubber Chemistry and Technology—Vol. 54—May/June 1981, n. 2, page 449).
Anisotropies are particularly critical during the service conditions of the electrical insulating element of the elastomeric sleeve. Moreover, once formed the anisotropies can not be adjusted or at least partially reduced since they remain “freezed” in the insulating material by the step of curing which immediately follows the step of filling the insulating material into the mould.
Therefore, the Applicant has perceived that the step of filling the insulating material into the mould remarkably influences the risk of breakdown of the insulating element of a joint for electrical cables.
In particular, the Applicant has perceived that, during the filling of the mould with the insulating material, the conveying thereof into the mould has to be carried out as uniformly as possible in order to substantially avoid the formation of said anisotropies and thus to reduce the risk of breakdown of the elastomeric sleeve.
More in particular, the Applicant has perceived that the formation of said anisotropies can be remarkably reduced by avoiding that the filling of the mould is carried out by causing each portion of insulating material to be pushed along the mould walls by the portions of insulating material entering the mould at successive time instants. The Applicant has perceived that this is particularly critical especially in proximity of the axial ends of the electrode where the electric field gradient reaches its maximum value and the disuniformity between the insulating material fed at the very beginning of the filling step and the insulating material fed at successive time instants is remarkably high.